1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a printer, and a fax machine, which performs image formation by an electrophotographic system.
2. Description of the Related Art
Conventionally, as an image forming apparatus such as a copying machine and a laser beam printer, an image forming apparatus having a configuration which uses an intermediate transfer member is known.
In this image forming apparatus, as a primary transfer process, a toner image formed on a surface of a photosensitive drum serving as an image bearing member is transferred onto an intermediate transfer member by applying a voltage from a voltage source to a primary transfer member arranged at a photosensitive drum opposing portion. After that, the primary transfer process is repeatedly performed for toner images of multiple colors, and thus the toner images of multiple colors are formed on the surface of the intermediate transfer member. Subsequently, as a secondary transfer process, the toner images of multiple colors formed on the surface of the intermediate transfer member are collectively transferred onto a surface of a recording material such as paper by applying a voltage to a secondary transfer member. The collectively transferred toner images are then permanently fixed to the recording material by a fixing unit. Thus, a color image is formed.
Japanese Patent Application Laid-Open No. 2011-232785 employs a configuration as follows. A roller opposing to the a photosensitive drum for primary transfer is formed of a rigid body such as a metal body, the opposing roller abuts against an intermediate transfer belt at a position shifted from a primary transfer portion, and the primary transfer portion is formed by bringing the intermediate transfer belt into contact with (increasing the contact area of the intermediate transfer belt with respect to) the photosensitive drum.
However, the above-mentioned conventional example has the following problem.
The intermediate transfer belt has large stretching characteristics with respect to tension. Therefore, when the intermediate transfer belt is driven to rotate, a stripe-like deformation may be seen on the belt surface in some cases. When this stripe-like belt deformation reaches an image forming portion, irregularities are generated at the primary transfer portion between the photosensitive drum and the intermediate transfer belt in its longitudinal direction. Depending on the place in the longitudinal direction of the primary transfer portion, an air gap is formed between the photosensitive drum and the intermediate transfer belt, which may cause a problem of image failure.